1. Field of the Invention
The present invention relates to a method of making a shaft.
2. Description of the Related Art
A steering shaft that transfers rotation of a steering wheel to steered wheels includes a member having a portion to absorb an excessive torsion load over a predetermined value when the torsion load is applied from the steered wheels side.
Japanese Patent Application Publication No. 2015-85805 (JP 2015-85805 A) describes an electric power steering system in which an intermediate shaft is coupled between an upper shaft coupled to a steering wheel and an input shaft coupled to the steered wheels via other members. The electric power steering system absorbs a torsion load by plastically deforming the intermediate shaft, when a torque that is equal to or higher than a reference value is applied from the steered wheels side.
With reference to FIG. 6A, a conventional intermediate shaft 512 will be described. As shown in FIG. 6A, the intermediate shaft 512 includes a spline portion 531 formed on a first side (right side in FIG. 6A) in an axial direction, a fitted portion 532 formed on a second side (left side in FIG. 6A) in the axial direction, and a neck portion 533 formed between the spline portion 531 and the fitted portion 532.
The spline portion 531 has an external spline formed in its outer periphery. The fitted portion 532 is shaped like a closed-end cylinder and has a concave portion 532a in its end surface. The outside diameter of the fitted portion 532 is larger than a maximum outside diameter of the spline portion 531. The neck portion 533 is a columnar portion having an outside diameter smaller than the outside diameters of the spline portion 531 and the fitted portion 532. When a torsion load over a predetermined value is applied, the intermediate shaft 512 absorbs the torsion load by plastically deforming the neck portion 533 in a torsion direction.
Next, with reference to FIGS. 6B and 6C, a method of making the intermediate shaft 512 will be described. In the first process, as shown in FIG. 6B, drawing is applied to part of a columnar material b having an outside diameter that is equal to the outside diameter of the fitted portion 532, in order to form a rough outer shape of the intermediate shaft 512.
To be specific, the drawing is applied to a first region b1 of the columnar material b extending from a predetermined position p in the axial direction to an end of the columnar material b on a first side in the axial direction. With this, a columnar shaft portion b11 and a tapered portion b12 are formed. The columnar shaft portion b11 extends from the end of the columnar material b on the first side (lower side in FIG. 6B) in the axial direction, toward the second side (upper side in FIG. 6B) of the axial direction. The tapered portion b12 is formed such that the outside diameter of the tapered portion b12 is increased as the tapered portion b12 extends from the shaft portion b11 toward the predetermined position p.
The outside diameter of the shaft portion b11 is equal to a maximum outside diameter of the spline portion 531. The number of times of the drawing is determined depending on a difference between the outside diameter of the columnar material b determined on the basis of the fitted portion 532 and the outside diameter of the spline portion 531. In the method shown in FIG. 6B, the drawing is applied three times.
After the drawing is applied, the columnar material b is subjected to the spline processing, as shown in FIG. 6C. The spline processing is performed on a portion of the shaft portion b11 that is to form the spline portion 531, and is applied to the outer periphery of the portion. Subsequently, cutting is applied to an end face of the columnar material b on the second side in the axial direction to form the concave portion 532a, and is applied to the outer periphery of the tapered portion b12 and the outer periphery of a portion of the shaft portion b11 near the tapered portion b12, to form the columnar neck portion 533. With this, an outer shape of the fitted portion 532 is formed on the second side of the neck portion 533 in the axial direction, and has a diameter larger than that of the neck portion 533.
In the above-described conventional method of making the intermediate shaft 512, one portion of the drawing-applied tapered portion b12, having a smaller outside diameter, has a higher work-hardening degree due to the drawing, and thus has a higher hardness. In contrast, another portion of the drawing-applied tapered portion b12, having a larger outside diameter, has a lower work-hardening degree, and thus has a lower hardness than the portion having the smaller outside diameter. As a result, the neck portion 533 has a large difference in hardness distribution, in the portion formed by the cutting of the outer periphery of the tapered portion b12.
Consequently, when an excessive torsion load over a predetermined value is applied to the conventional intermediate shaft 512, the neck portion 533 is twisted locally at a portion having a lower hardness (i.e. larger outside diameter). Thus, it is difficult to twist the neck portion 533 as a whole in the axial direction. As a result, the conventional intermediate shaft 512 cannot absorb the torsion load sufficiently.